A plurality of conduits or pipes of relatively small diameter typically extend from a seabed structure to a service platform and are enclosed within an outer ‘carrier’ pipe. Such an arrangement is known as a ‘pipeline bundle’, and is adapted to convey produced fluids from the well.
Additionally having flowed along the pipelines or bundles, hydrocarbons are then transported to the surface for processing via vertical structures which can be either single pipes or a plurality of pipes. Such an arrangement is known as a ‘riser’ and is adapted to convey fluids from the seabed to the surface for processing and treatment.
When fluids are extracted from subsea reservoirs, their temperature is normally higher than that of the surrounding water. As the fluids cool in the pipes to the temperature of the water, certain compounds come out of solution, and this causes problems with precipitates such as waxes or gas hydrates that build up within the pipelines and can reduce or prevent the flow of hydrocarbons. Moreover, the viscosity of the produced fluids increases as their temperature decreases, which also reduces the rate of flow through the pipeline. Additionally if gas comes out of solution it can, under certain conditions of pressure and temperature, combine with water to form clathrate structures known as ‘Hydrates’ which can accumulate within the pipeline causing blockages reducing or preventing the flow of hydrocarbons.
It is known to insulate pipeline bundles in several ways to offset these problems—for instance pipes are often insulated using solid syntactic foam preformed insulating coatings. Alternatively the pipe-in-pipe annular spaces are evacuated, or are filled with silica-based materials or hollow spheres of plastic material in a synthetic resin matrix.
These and similar techniques have serious disadvantages in the way of investment cost, difficulty of handling, and the requirement for specialist equipment to manufacture the materials, and as a consequence the materials needed for the techniques are not routinely made up at the point of manufacture of the pipeline. Also some of the materials needed for the insulation are limited with respect to the depth of water in which they can be applied. For example, at depths beyond around 1500 metres the hydrostatic pressure of the water column will collapse foam and its insulating qualities will be lost.
A further attempt to solve this problem is disclosed in European Patent Publication No 0,336,493. In this application, a liquid hydrocarbon gel is provided in the outer pipe to insulate the smaller diameter pipes running therethrough. However, the use of a fluid gel requires the provision of a pressure balancing system which is prone to failure.
Other subsea structures, such as trees provided at a wellhead, can also require insulation.